1. Field of the Invention
This invention relates to an automatic vehicle identification system, an interrogator and responder included in the same, the interrogator recognizing the responder by transmitting an interrogating signal and receiving a responding signal from the responder to identify the responder.
2. Description of the Prior Art
Automatic vehicle identification systems are known. Such automatic vehicle identification systems have interrogators and responders. The interrogator identifies a responder by transmitting an interrogating signal and receiving a responding signal from the responder.
In one of such automatic vehicle identification systems, the interrogator transmits the interrogating signal through a microwave radio signal. A responder entering a service area of the interrogator receives the interrogating signal and effects a spectrum coding with a predetermined data. Then, the responder transmits the responding signal to the interrogator. The interrogator can recognize the responder because the interrogator has the predetermined data also and decode the responding signal with the data.
FIG. 7 is a block diagram of such a prior art automatic vehicle identification system. In FIG. 7, an interrogator 401 continuously transmits an interrogating signal X obtained by modulating an oscillation signal from an oscillator 440 with predetermined data by the modulator 441 through an transmitting antenna 442. When a responder 404 enters a service area of the interrogator 401, the responder 404 detects the presence of the interrogating signal X received by a receiving antenna 443 and then, spread-spectrum-codes the interrogating signal X with an identification code stored in an identification code memory 444 provided thereto and transmits a spread-spectrum-coded interrogating signal X as a responding signal Y through a transmission antenna 446. The interrogator 401 receives the responding signal Y and demodulates the received responding signal Y with one of identification codes read from a memory table 447 provided therein and detects a reproduction condition of the demodulated response signal. When the reproduction condition is not sufficient, that is, synchronization between the interrogator and the responder is not obtained, the read identification code used in the responder 404 does not agree with the identification code read from the memory table 447. Then, the interrogator 401 reads another identification data from the memory table 447 and demodulates the received response signal. This operation is repeated until the reproduction condition of the received response signal is sufficient. When the reproduction condition is sufficient, the interrogator can identifies the responder and produces an output signal 450 obtained by demodulating the received response signal by the spread spectrum demodulator 449.
However, in the above-mentioned prior art, there is a problem that the spread spectrum modulator 445 is necessary in the responder 404 though the responder 404 is required to have a small size and a low power consumption.
However, in the interrogator 401, a circuit for identification between the interrogator 401 and the responder 404, so that sizes of the interrogator 401 was large.